Diaptomus oregonensis. 449 



18 m. , almost no Diaptomi were found. The maximum number 

 from this level was 13, and in about one-fourth of the hauls 

 none were found. The average was 4. The 12-15 m. level con- 

 tained scarcely more, having an average of 6 and a maximum of 

 15. Nine hauls out of fifty-five showed no result. On July 21, 

 seven observations were made in the deeper parts of the lake at 

 widely separated points, where dredgings could be made down 

 to a depth of 22 m. or a little more. The same conditions were 

 found to exist here. Below 15 m. almost no Diaptomi were 

 found. The average of the 7 m., 15-22, was only about 3, with 

 a maximum of 8. Similar observations were made on other 

 days, with precisely the same result. It is therefore plain that 

 the region of water below 12 m. was practically without 

 Diaptomi. On the average more than 99 per cent, of the species 

 was found in the upper 12 m. of the lake. 



The 9-12 m. level contained more, — a little over 3 per cent. 

 of the whole catch. The number for the whole time averaged 

 133, and varied from 2 as a minimum to 570 as a maximum. 

 In the different periods the number in this level also varied. 

 In Period II the average number was 50, with an average vari- 

 ation of 49 per observation. In Period III the average was 157, 

 with an average variation of 97. In Period IV the average was 

 224, with an average variation of 175. The number of Diap- 

 tomi in this level seems to vary with the degree of disturbance 

 of the lake, being smallest in protracted calm weather such as 

 characterized Period II, and greatest in the stormy weather of 

 Period IV. Measured in percentage of the total catch the high- 

 est number noted was 12.6 per cent., which was found at 3 a. m. 

 of August 4. 



On an average by periods from 92 to 97 per cent, of the 

 Diaptomi were found permanently in the upper 9 meters of the 

 lake during July. The maximum number found in this region 

 was over 99 per cent., the minimum 86.5 per cent. 



The vertical distribution is well differentiated in the three 



levels into which the upper 9 m. were divided. In the upper 



(0 — 3 m.) level was found on the average 52.8 per cent., with 



a maximum of 80 (J. 17, 9 p. m.) and a minimum of 31 per 



Cent, found on two occasions (J. 27, 9 p. m. ; A. 3, 9 a. m.). 



